This Application claims priority from PCT/IT98/00039, filed Feb. 25, 1998.
1. Field of the Invention
The present invention relates to an envelope for transparencies for overhead projectors and the like. More specifically, the present invention is a transparency envelope having a transparency contact surface including a plurality of integral micromounds. The envelope of the present invention offers superior optical performance by reducing or eliminating Newton""s rings, interference fringes and visible oil pooling effects, as well as offering desirable clarity, weight and appearance.
2. Description of the Prior Art
The use of overhead projectors and the transparencies associated therewith has been amply documented. The use and display of graphics and text in business presentations, lectures and the like often augments the effectiveness of the speaker and of the presentation. Transparencies, bearing graphics and/or text, are often exhibited to the viewers on a screen using an overhead projector. Transparencies allow the presenter to preview the presentation, easily sort through materials, point out specific features, and easily add additional comments or graphics. Newer transparency designs offer compatibility with printing devices, and thus may be printed without the need of specialized equipment.
However, individual transparencies are delicate. The transparent film, coatings and inks may be easily damaged in storage or transit. Also, dirt and dust may deposit over the image. Any blemishes to the image are, by the very nature of the medium, amplified and highlighted during a presentation.
Storage and projection accessories have been developed to protect the transparencies. A variety of transparency protection devices such as protective envelopes, frames, cover sheets, transparency assemblies and the like that contain or overlay the transparency have been developed. A number of such protective accessories include transparent sheets of material, which are placed directly against the surface of the transparency. Industry practice has been to use both polyvinylchloride (PVC) and polyethyleneterephthalate (PET) in the manufacture of the transparent sheets. Polypropylene and cellulose acetate, have been suggested for the same purpose.
Certain problems have arisen with the use of imaged film sheets inserted in transparent film envelopes, among them the appearance of Newton""s rings, interference fringes, and oil pooling.
Newton""s rings and interference fringes are optical phenomena observed when two sheets approach close to one another, and the small air gap between the sheets has a different refractive index from that of the sheet materials. The phenomena are better observed when one or both of the sheets is transparent and are due to the wave nature of light. Detailed explanations of the theory of the phenomena may be found in, for example, Warren J. Smith, Modern Optical Engineering, p. 14 (McGraw-Hill, 2nd ed. 1990), and in Francis Arthur Jenkins, Fundamentals of Optics, xc2xa714.4, p. 294 (McGraw-Hill 1976).
Newton""s rings are evidenced when two surfaces touch locally, the point of contact being surrounded by a series of rings whose widths increase with distance from the point of contact. In appearance, Newton""s rings consist of concentric contour bands exhibiting, in white light, a change in color across the band. In monochromatic light, these concentric contour bands are uniform in color, but graded in intensity across the band.
Interference fringes are similar in origin to Newton""s rings, but are observed when two surfaces make contact along a line. In white light a series of colors bands may be observed approximately parallel to the line of contact. In presentations, Newton""s rings and interference fringes formed between a protective sheet and a transparency may be obtrusive and undesirable.
Oil pooling is a consequence of the use in certain copying or printing equipment of so-called silicone oils. As the desire for faster copiers has arisen, the need has been met by raising the operating temperature of fuser rolls. The application of a higher temperature for a shorter period of time has enabled copier speeds to rise. In addition certain color copiers require extreme fusing conditions to produce adequate color transparency. Problems have been encountered with toners or sheets adhering to the fuser rolls, and perhaps causing a serious breakdown of the copier. To reduce adhesion of the film and or toner to the fuser roll, copiers have been designed that provide a thin film of silicone oil on the surface of the fuser roll.
A certain amount of the oil remains on the film surface and results in somewhat unpleasant handling characteristics. Handling of the transparency may be reduced by placing the transparency in a film envelope. However, a detrimental side effect arises in that the residual oil is present in a sufficient amount to wet both the film envelope and the transparency, and flow to form localized concentrations of oil into a film of oil. The resulting oil pooling film has noticeable refractive properties and is considered objectionable and obtrusive on a projected image.
Particulates have been added to imageable coatings to alleviate oil pooling effects. However when particulate of a sufficient concentration and particle size is incorporated to eliminate the oil pooling problem, the particles are visible and cause unacceptable haze.
The need remains for protective sheets that eliminate or reduce Newton""s rings, fringe interference and oil pooling.
The present invention is directed to a protective sheet and to transparency protection devices that reduce or eliminate Newton rings, interference fringes, and oil pooling effects without adding significant additional weight and while improving the overall appearance of the transparency packages. A film envelope for protecting a transparency, in accordance with the present invention includes a substantially rectangular pocket having at least one generally transparent sheet. The sheet comprises a film having a first and a second major surface, wherein at least a portion of one major surface includes a plurality of micromounds protruding from the major surface.
In one embodiment, the sheets comprise micro-textured bi-axially oriented polypropylene. In an alternative embodiment, the portion of the major surface including a plurality of micromounds comprises a micro-textured surface having a particulate and a multilayer film material. An additional anti-scratch surface coating may be applied to at least a portion of one of the major surfaces.
The micromounds have a height ranging between 1 to 10 microns, a micromound density in the range of 10 to 500 micromounds per square millimeter, and a major axis length in the range of 5 to 100 microns.
In a preferred embodiment, the micromounds have a height ranging between 2 to 5 microns, a micromound density in the range of 100 to 300 micromounds per square millimeter, and a major axis length in the range of 20 to 60 microns.
The envelope may further comprise one or more flaps, perforation holes, frames or other transparency protection, storage, and presentation elements known in the art.